Technical Abstract:
The production and sales of ornamental plants are a large component of the worldwide economy. Introduction of new and improved ornamentals commonly occur via selection, but plant breeders are continually striving to develop new plants with superior ornamental qualities via conventional breeding. Breeding woody ornamental trees presents unique challenges, such as long juvenility periods, obligate or nearly obligate outcrossing breeding systems, and limited histories or pedigrees regarding genetic improvement. However, recent advances in molecular technologies, including relatively inexpensive genome sequencing, construction of various nucleic acid libraries, mapping of markers, and a myriad of profiling techniques, make discovery and utilization of molecular markers for desirable traits possible in specialty crops such as ornamental plants. To obtain more information on the flowering dogwood genome, a single custom, normalized library for Roche/454 sequencing was prepared and sequenced for bract (‘Cherokee Brave’) and leaf (‘Appalachian Spring’ and ‘Cherokee Brave’) tissue. The 1,621,644 reads were assembled into 63,029 unique sequences, comprising 42,946 isotigs (26,627 isogroups) and 20,083 singletons. Also, 5,670 expressed sequence tags (EST) – simple sequence repeats (SSRs) were identified from the data set. To demonstrate the usefulness of this EST resource for marker development, primers were designed for 2,998 of the EST-SSRs of which 94 were tested for amplification of loci in ‘Appalachian Spring’, ‘Cherokee Brave’, and two F1 breeding lines. Seventy-five of the loci amplified and 29 were polymorphic, from which 9 of the most polymorphic loci were selected and tested using 22 individuals representing 5 dogwood species. The number of alleles detected ranged from 3 to 10 and observed heterozygosity was 0.05 to 0.67. Cross species transfer was higher in the big – bracted dogwood species (C. elliptica and C. kousa) than in the non – bracted species (C. alternifolia and C. sericea) and principal coordinate analysis differentiated all of the species. This work confirms the applicability of flowering dogwood EST-SSRs for the breeding and genetics of flowering dogwood and other Cornus species. We expect to optimize more EST-SSRs in order to accelerate breeding of flowering dogwood cultivars with improved disease resistance, heat or drought tolerance, and other aesthetically appealing ornamental traits.